Optical integrated circuits and optoelectronic integrated circuits are usually fabricated on the top surface of a uniform substrate. These circuits can experience optical losses as light propagates away from the circuits and into the uniform substrate. It is desirable to produce optical and optoelectronic integrated circuits as compact as possible on a substrate to produce small lightweight circuits. However, losses experienced by the interconnects between circuits tend to inhibit the reduction of the overall system size.
Interconnects in optical integrated circuits and optoelectronic integrated circuits are achieved by using waveguides to transport light from one optical device to another. Waveguides are also the basis of numerous optical devices including optical couplers, switches, modulators, power dividers and combiners.
Light propagating through an optical waveguide is contained within the waveguide by total internal reflection. The medium outside the waveguide has a lower index of refraction than does the interior of the waveguide. The boundary between the interior and exterior of the waveguide is characterized by a critical angle determined by the ratio of the refractive indices or index contrast of the two media. The critical angle of the boundary can be defined as the angle below which light inside the waveguide must strike the boundary in order to be reflected back into the waveguide rather than be transmitted through the boundary and out of the waveguide. Light inside the waveguide which strikes the boundary at an angle smaller than the critical angle cannot pass through the boundary and is reflected back into the waveguide.
Since nearly all of the light traveling through a straight optical waveguide strikes the waveguide boundary at very small angles, very little loss is suffered. However, in fabricating optical and optoelectronic integrated circuits, it is necessary to make optical waveguides with bends. Also, certain devices such as optical couplers require bends in their waveguides. Unlike straight waveguide sections, waveguide bends can produce significant losses. As the light traveling through the waveguide enters a bend, it strikes the boundary at a larger angle than it does in a straight section. If the index contrast does not provide a large enough critical angle, some of the light escapes from the waveguide, thus introducing bend loss.
Bend losses have proven to be a substantial impediment to the development of optical and optoelectronic integrated circuits. To minimize losses, bends must be made with large radii of curvature, typically on the order of 10 mm. Such large bend radii are not practical for compact optical and optoelectronic integrated circuits.
Numerous approaches have been suggested to minimize bend losses. One of these is the use of abrupt bends rather than curved bends. Abrupt bends have sharp corners joining two straight sections of waveguide. The drawback to abrupt bends is that the bend angle must be very small, on the order of 1 degree. So, this approach contributes little to making circuits more compact.
Another approach has been to grade the index of refraction of the substrate in the area of the bend to maintain internal reflection around the bend. Combinations of these two approaches have also been suggested. However, no present approach appears to be able to substantially reduce the radii of curvature of waveguide bends to facilitate the development of compact optical and optoelectronic integrated circuits.